Clocks are commonly used in various electronic systems and circuits to provide timing references. They are used in numerous integrated circuits (IC's) such as microprocessors and data converters, and typically are required in any communication or telecommunication systems. Many different methods exist for generating a clock signal. Oscillators in one form or another are typically used, and may for example include a Schmitt trigger and an RC network, or op amps or comparators with positive feedback, or a crystal which oscillates when a voltage is applied, with or without a phase-locked loop (or PLL). Likewise, a free-running ring oscillator might be used, typically constructed of an odd number of inverters or similar single gain stages. Each approach has its own advantages and drawbacks. Ring oscillators and Schmitt trigger oscillators are smaller and easier to implement than crystal (or PLL) circuits. However, crystal (and PLL) based clocks have highest accuracy of the oscillation frequency. So, the accuracy of clock is often improved at the expense of increased designs complexity, cost and power consumption.
Regardless of the design for a clock, duty cycle and frequency are two important elements. Referring to FIG. 1, duty cycle of a clock is defined asD=THigh/T=THigh/(THigh+TLow)  (1)where T is the total period of the clock and its related to its frequency f by T=1/f, THigh is the duration of clock within a period T for which the clock is at logic 1, and TLow is the duration of clock within a period T for which the clock is at logic 0. Typically, frequency of a clock can be controlled independent of its duty cycle and its duty cycle can be regulated separate from its frequency by various methods. However, the prior art has not permitted simultaneous variation of both frequency and duty cycle, which permits, among other things, adjustment for operating conditions.